GB 1,140,167 discloses a machine of that type, in which the synchronization system comprises a drive part that is stationary relative to the sliding disk and that, for each coupling rod, presents a recess through which the coupling rod passes, said recess being in the form of a radial slot that is open onto the outer periphery of the drive part. While the cylinder block is rotating, the coupling rod that passes through a recess comes intermittently into contact with the faces of the slot that forms the recess, thereby making it possible to hold said rod in a position such that its axis is approximately contained in a normal radial plane, containing the second axis of rotation and a radius extending from said axis and passing through the center of the second spherical joint corresponding to said rod. Thus, the axis of each coupling rod is held approximately in a normal radial plane, so that the rotation of the sliding disk about the second axis of rotation is synchronized with the rotation of the cylinder block about the first axis.
The time taken for synchronization, between the moment when, under the effect of the cylinder block rotating, a coupling rod tends to depart from a position in which its axis is contained in the normal radial plane that contains the center of its second spherical joint and the moment when such departure is countered by the contact between the rod and a face of the slot, thereby holding the axis of said rod approximately in said normal radial plane, is a function of the respective dimensions of the slot and of the rod engaged therein. More precisely, said time depends on the reference clearance between the first drive surface formed on the rod and the second drive surface formed by the wall of the slot, said reference clearance being the clearance that is measured between said surfaces when the axis of the coupling rod is in its normal radial plane.
In GB 1,140,167, the slots in the drive part serve to accommodate the tangential movements of the coupling rods, but, insofar as said slots are open onto the outer periphery of the drive part, said tangential movements are not limited when the radial movements increase.
In the description below, the tangential direction is considered to be the direction that is tangential to the circle described by the centers of the second spherical joints whereas the radial direction is the direction that is radial relative to said circle.
Patent Application PCT/EP2004001560 (published as WO2005/078238) discloses a synchronization system in which the first and second drive surfaces are each formed by rotating a generator line about an axis and are thus “surfaces of revolution” or “rotational surfaces”. As is explained in that patent application, this feature makes it possible to reduce the synchronization times by limiting the distance between a first drive surface and the corresponding second drive surface.
As indicated above, the synchronization system serves to hold the axis of each coupling rod approximately in its normal radial plane, i.e. to ensure that the centers of the second spherical joints are positioned correctly and to reduce the forces acting on the coupling rods.
For each coupling rod, clearance is necessary between the first drive surface and the second drive surface. While the cylinder block is rotating, the coupling rod tends to pivot relative to the center of the first spherical joint. This tendency to pivot results from the fact that the second axis of rotation is inclined relative to the first axis of rotation. The centers of the first spherical joints are disposed on a first circle centered on the first axis of rotation and contained in a plane perpendicular to said axis, whereas the centers of the second spherical joints are disposed on a second circle centered on the second axis of rotation and contained in another plane perpendicular to said axis. Due to the inclination between said axes, the projection of the first circle onto the plane containing the second circle forms an ellipse. As a result, while the cylinder block is rotating, the axis of each coupling rod substantially describes a cone whose vertex is at the center of the second spherical joint, assuming that the axis of the coupling rod is a straight line passing through the centers of the first and second spherical joints.
Thus, while the cylinder block is rotating, the first and second coupling surfaces come intermittently into contact with one another. If it is considered that a coupling rod is initially in a position in which its axis is in its normal radial plane, the rotation of the cylinder block tends to tilt the rod which thus moves away from this initial position until the first drive surface comes into contact with the second drive surface, thus tending to constrain the cylinder block and the sliding disk to rotate together instantaneously, and thus to synchronize them.
The angle between the axis of the coupling rod and the second axis of rotation is designated by angle β below.
With the synchronization system, the aim is for the angle of inclination β of the coupling rods to remain small, while accommodating the angular movements of the coupling rods that are necessary, as indicated above, due to them pivoting relative to the centers of the first spherical joints.
For a coupling rod under consideration, the angle β varies while the cylinder block is rotating. The distance between the second drive surface and the first drive surface is such that said second drive surface comes periodically into contact with the second drive surface, when the angle β reaches a value such that said contact is established. The force exerted by the second drive surface on the first surface during said contact is referred to below as the “synchronization force”.
The synchronization forces depend on the clearance between the drive surfaces, on the angle of inclination of the swash plate, i.e. on the angle of inclination between the second axis of rotation and the first axis of rotation, and on the elasticity of the material of which the coupling rods are made.